ARS Scientists Study Holstein Milk Production,
Fertility

Agricultural Research Service (ARS)
scientists have discovered why Holsteinsbred to produce more
milkare less fertile than before breeding efforts were stepped up to
increase dairy production: It's in their DNA.

Since the U.S. dairy industry intensified selective breeding efforts in the
1960s, average milk yield in Holsteins has doubled, but the cattle are less
fertile. A comparison of DNA from cattle selectively bred for milk production
versus cattle isolated from such practices shows a genetic link between
increased yields and reduced fertility, according to researchers at the ARS
Animal
and Natural Resources Institute (ANRI) in Beltsville, Md.

The researchers teamed up with colleagues at the
University of Minnesota
(UM) to compare the genomes of modern Holsteins with those of UM cattle never
exposed to the modern selective breeding practices. The lack of exposure meant
that DNA from the UM cattle were genetic "time capsules" of an era
before the selection efforts intensified.

ANRI geneticist
John
Cole and colleagues drew DNA samples from the genetic material of about
2,000 cattle, stored at the
ARS
National Center for Genetic Resources Preservation, the
Holstein Association USA, and five U.S.
universities, including UM. The scientists extracted DNA and genotyped the
samples using the Illumina Bovine SNP50 BeadChip, a glass slide capable of
obtaining genotypes for thousands of markers simultaneously. The device was
developed by ARS researchers in collaboration with industry, university and
other ARS partners.

By analyzing 50,000 genetic markers, the researchers found that many of the
chromosomal regions associated with increased milk yield were also associated
with reduced fertility rates. The results also showed that up to 30 percent of
the Holstein genome may be influenced by standard breeding practices, according
to Tad
Sonstegard, an ANRI geneticist.

The researchers say the results will help Holstein breeders and milk
producers better understand tradeoffs between high yield and low fertility when
selecting for more profitable dairy cattle.

Read
more about this research in the October 2009 issue of Agricultural
Research magazine.